Electrostatic printers of this type eject charged solid particles dispersed in a chemically inert, insulating carrier fluid by using an applied electric field to first concentrate and then eject the solid particles. Concentration occurs because the applied electric field causes electrophoresis and the charged particles move in the electric field towards the substrate until they encounter the surface of the ink. Ejection occurs when the applied electric field creates an electrophoretic force that is large enough to overcome the surface tension. The electric field is generated by creating a potential difference between the ejection location and the substrate; this is achieved by applying voltages to electrodes at and/or surrounding the ejection location.
The location from which ejection occurs is determined by the printhead geometry and the location and shape of the electrodes that create the electric field. Typically, a printhead consists of one or more protrusions from the body of the printhead and these protrusions (also known as ejection upstands) have electrodes on their surface. The polarity of the bias applied to the electrodes is the same as the polarity of the charged particle so that the direction of the electrophoretic force is away from the electrodes and towards the substrate. Further, the overall geometry of the printhead structure and the position of the electrodes are designed such that concentration and then ejection occurs at a highly localised region around the tip of the protrusions.
The ink is arranged to flow past the ejection location continuously in order to replenish the particles that have been ejected. To enable this flow the ink must be of a low viscosity, typically a few centipoises. The material that is ejected is more viscous because of the higher concentration of particles due to selective ejection of the charged particles; as a result, the technology can be used to print onto non-absorbing substrates because the material will not spread significantly upon impact.
Various printhead designs have been described in the prior art, such as those in WO 93/11866, WO 97/27058, WO 97/27056, WO 98/32609, WO 98/42515, WO 01/30576 and WO 03/101741.
WO 98/42515 proposes a system for controlling the application of first voltage pulses to a respective ejection electrode associated with an ejection location and second voltage pulses to a respective secondary electrode associated with the ejection location, such that, when a voltage pulse is applied to the ejection electrode, a voltage pulse, inverted with respect to the pulse applied to the ejection electrode, is applied to the secondary electrode. This technique is used to overcome the capacitive coupling between proximate ejection locations which otherwise can adversely effect ejection. This coupling can be reduced if lower voltages are used, and it is therefore desirable to use the smallest possible voltages to cause ejection. Inverting the voltage applied to the secondary electrode maintains the differential voltage at a desirable level while reducing the amplitude of the voltage change required on each electrode. The provision of voltages on secondary electrodes of this type also serves to preserve a symmetrical electrical field shape which minimises the deflection (side-to-side) resulting otherwise from asymmetrical fields arising from the voltages applied to adjacent ejection locations.
FIG. 1 is a drawing of the tip region of an electrostatic printhead 1 of the type described in this prior art, showing several ejection upstands 2 each with a tip 21. Between each two ejection upstands is a wall 3, also called a cheek, which defines the boundary of each ejection cell 5 or ejector. In each cell, ink flows in the two channels 4, one on each side of the ejection upstand 2 and in use the ink meniscus is pinned between the top of the cheeks and the top of the ejection upstand. In this geometry the positive direction of the z-axis is defined as pointing from the substrate towards the printhead, the x-axis points along the line of the tips of the ejection upstands and the y-axis is perpendicular to these.
FIG. 2 is a schematic diagram in the x-z plane of a single ejection cell 5 in the same printhead 1, looking along the y-axis taking a slice through the middle of the tips of the upstands 2. This figure shows the cheeks 3, the ejection upstand 2, the ejection location 6, the location of the ejection electrodes 7 and the position of the ink meniscus 8. The solid arrow 9 shows the ejection direction and also points towards the substrate. Typically, the pitch between the ejection cells is 168 μm. In the example shown in FIG. 2 the ink usually flows into the page, away from the reader.
FIG. 3 is a schematic diagram of the same printhead 1 in the y-z plane showing a side-on view of an ejection upstand along the x-axis. This figure shows the ejection upstand 2, the location of the electrode 7 on the upstand and a component known as an intermediate electrode (abbreviated to IE) (10). The intermediate electrode 10 is a structure that has electrodes 101, on its inner face (and sometimes over its entire surface), that in use are biased to a different potential from that of the ejection electrodes 7 on the ejection upstands 2. The intermediate electrode 10 may be patterned so that each ejection upstand 2 has an electrode facing it that can be individually addressed, or it can be uniformly metallised such that the whole surface of the intermediate electrode 10 is held at a constant bias. The intermediate electrode 10 acts as an electrostatic shield by screening the ejection location/ejector from external electric fields and allows the electric field at the ejection location 6 to be carefully controlled.
The solid arrow 11 shows the ejection direction and again points in the direction of the substrate. In FIG. 3 the ink usually flows from left to right.
In operation, it is usual to hold the substrate at ground (0 V), and apply a voltage, VIE, between the intermediate electrode 10 and the substrate. A further potential difference of VB is applied between the intermediate electrode 10 and the electrodes 7 on the ejection upstand 2 and the cheeks 3, such that the potential of these electrodes is VIE+VB. The magnitude of VB is chosen such that an electric field is generated at the ejection location 6 that concentrates the particles, but does not eject the particles. Ejection spontaneously occurs at applied biases of VB above a certain threshold voltage, VS, corresponding to the electric field strength at which the electrostatic force on the ink exactly balances the opposing force from the surface tension of the ink. It is therefore always the case that VB is selected to be less than VS. Upon application of VB, the ink meniscus moves forwards to cover more of the ejection upstand 2. To eject the concentrated ink, a further voltage pulse of amplitude VP is applied to the ejection upstand 2, such that the potential difference between the ejection upstand 2 and the intermediate electrode 10 is VB+VP. Ejection will continue for the duration of the voltage pulse. Typical values for these biases are VIE=600 V, VB=1000 V and VP=300 V.
The voltages applied during print operation may be derived from the bit values of the individual pixels of a bit-mapped image to be printed. The bit-mapped image is created or processed using conventional design graphics software such as Adobe Photoshop and saved to memory from where the data can be output by a number of methods (parallel port, USB port, purpose-made data transfer hardware) to the print head drive electronics, where the voltage pulses which are applied to the ejection electrodes of the printhead are generated.
Printheads comprising any number of ejectors can be constructed by fabricating numerous cells 5 of the type shown in FIGS. 1 to 3 side-by-side along the x-axis. A controlling computer converts image data (bit-mapped pixel values) stored in its memory into voltage waveforms (commonly digital square pulses) that are supplied to each ejector individually. By moving the printhead 1 relative to the substrate in a controllable manner, large area images can be printed onto the substrate.
The electric field at the ejection points of the printhead, or of the channel, normally acts to force the colorant particles suspended in the ink towards the ejection point, creating a concentration of particles at the ejection point. When a channel prints, this concentration of particles is ejected from the head to the substrate. However, if a channel is not required to print for a period of time, evaporation of the carrier fluid may cause a layer of concentrated ink to form at the ejection point which can impede the ejection of ink when a print voltage is next applied to the channel for print operation.
This may result in a printhead being slow to respond to the start of printing an image. This effect is stronger if the bias voltage of the printhead has been on for a period of time prior to the start of printing.
Accordingly, there is a need to provide a method for reducing and/or preventing the above process which occurs when the printhead has not printed for a period of time, and an electrostatic inkjet printer which is not susceptible to said effect.